The optimal analysis of a visual scene requires the efficient processing and representation of the objects in the scene as well as their spatial positions. There is, however, far too much information to be represented at any one point in time with the result that these multiple possible locations and/or objects compete for neural representation. Given this limitation, people are perceptually selective, experiencing and responding to only a subset of the sensory signals evoked by events in the local environment. Selection can occur from representations of spatial location but also from representations of the objects or features in a scene and can be determined top-down by the current behavioral goals of the observer or bottom-up by the properties of the stimulus. The psychological and neural mechanisms that mediate perceptual selectivity are termed "attention," and the focus of this proposal is on the nature and operation of these attentional mechanisms. The research adopts a multidisciplinary approach to the investigation of visual attention by combining behavioral, functional imaging and anatomical studies in normal and brain-damaged individuals. Three major questions are addressed. The first concerns the mechanisms underlying selection by spatial location and the role of parietal cortex (and subdivisions therof) in modulating selection in a top-down and bottom-up fashion. The second question parallels the first but investigates mechanisms associated with object-based selection and the hemispheric specialization of parietal cortex. The final question addresses the interaction of space and object-based attention in order to understand the unitary and seamless nature of perception. In addition to shedding light on the mechanisms of attention, this research examines how brain damage affects attentional function. Understanding the intact system (and its breakdown) will enable us to develop effective methods for remediation. Acquired attentional deficits are notoriously difficult to rehabilitate, with profound adverse consequences for the individual, and targeted intervention efforts are urgently needed.